#!/usr/bin/env perl

use strict;
use warnings;

my ($x, $y) = split / /,<>; chomp $y;
my $count = 0;
for (my $i = $x; $i <= $y; $i++) {
  my $ave = ($i + $x + $y) / 3;
  if ($ave == int($ave)) {
    $count++;
  }
}
print $count;